A moving direction of a sliding block is restricted by geometrical restraint surfaces of guiding rail parts of a slide rail cooperated with rolling balls or needle rollers. The sizes of the guiding rail part, the rolling ball (or the needle roller), and guiding grooves of a sliding block are related to the pre-loading force and the overall smoothness. The conventional method is to perform a friction quality test after the slide rail is assembled by preparing a plurality of rolling balls (or needle rollers) in different size. If the result is nonqualified, the size would be modified or the rolling balls (or the needle roller) would be replaced with another size, and the friction test would be performed repeatedly until it is qualified. However, this might increase manufacturing complexity and increase the cost of material, time cost and workforce.
Other tests may include: using a coordinate-measuring machine, a profilometer or an optical type measurement device. However, the coordinate-measuring machine is too big in size, so it is not suitable for online testing. When a moving direction of a probe of the profilometer is perpendicular to a scanning direction, the probes would be stuck due to lateral force. The accuracy of the optical type measurement device is decreased by the influence of cutting oil and dust.